… In general, we should guard our thoughts-speech-actions, refraining from worrisome and disheartening thoughts. We should go in the opposite direction, [thinking,] speaking and doing things that lift us up and bring us happiness. This behavior will settle and become part of one’s mindset. And in this way, [as a result] a spirit from above will shower joy and an upbeat heart upon us. And this is what I heard from my grandfather, our teacher and Rebbe [The Baal haTanya]:
The Magid* (Nishmoso Eiden) would say, on the Pasuk [Yechezkel 1,26] “… like the appearance of a man upon it, from above,” that in accordance with how a person projects himself down in this world, so they relate to him from above. Therefore, he** recommended that I should refrain from singing sad*** tunes during davening. He waited until I finished davening Maariv and then told me the [above] Torah of the Magid. …
==== Igros Kodesh Tzemach Tzedek P21 22.
*The Magid of Mezritch, took over the leadership of the Chassidim after the Baal-Shem-Tov. The Alter Rebbe/ Baal haTanya was his talmid.
** The Baal haTanya /Chabad Rebbe, maternal grandfather of the Tzemach Tzedek/ the third Chabad Rebbe. The Tzemach Tzedek would hum tunes during his tefillos.
*** Not to confuse sad tunes with slow, passionate tunes.
Note: The above Torah of the Mezritcher Magid, the Alter Rebbe/Baal haTanya said to the Tzemach Tzedek on the last Maariv before the Alter Rebbe was nistalek. The Alter Rebbe was nistalek on Motzaei Shabbos, 24th of Teves 5573/1812.